I am in the UK using Basecamp with Windows 7. I downloaded the OSM map of Western Australia and Singapore combined from openstreetmap.nl into my SDHC card in my etrex 20 and also installed it in Basecamp.

In Basecamp I can see roads and lots of symbols, which make the map look like a very comprehensive map of WA and Singapore.

I have enabled only that map in etrex 20 but when I view it I can see only green and beige areas and my tracks and waypoints saved during previous visits but none of the roads and normal map information which is visible using the same map in Basecamp.

I don't know why I can't see the data in my etrex 20. I would be grateful for your help.

Thanks for your suggestions. I have been trying them out. In Basecamp all maps show all the details one could wish for.In my etrex 2o with the "Most" detail setting the same maps from garmin.openstreetmap.nl (for Western Australia and Singapore) and Talkytoaster(UK map)show rivers, names of towns, outlines of buildings, symbols for woods, etc but do not show roads (not even the M25 in the UK). Freizeitkarte_AUT for Austria shows the same on etrex 20 as in basecamp - roads, footpaths, woods, lakes, rivers and everything else. Enabling the map I want to see and disabling every other map makes no difference. It's very strange and I would be very grateful for your help.

I don't experience such problems with my etrex (30). You could try a factory rest on your device to see if you misconfigured something but I can't image how a bad configuration should lead to hidden roads.

I tried the factory reset using the menu in the etrex 20. → That made no difference at all with the problem of not seeing roads and footpaths, except for the Freizeitkarte_AUT where I could see all details anyway.

Then I discovered the "Master reset" procedure (press and keep pressing the Menu and toggle buttons and power up the etrex, keep pressing menu & toggle buttons until "Do you really want to...." is displayed, then "Yes" and it reboots). → And there were the roads, footpaths, everything I wanted. Magic.

The map (.img) that you downloaded will have been created from older data. your download may have a creation date with it and if you wished by using one of the map editors such as Potlatch2 or JOSM you can check the date of any thing that now appears but does not show on your map.

Another reason could be that the creator of the download only wished to map a limited amount of information, such as main roads or cycle ways. If you tell us where the download is from, someone who knows that map may tell you.
Reading your question again, and as you can see good coverage in basecamp, it may be that the etrex 20 as a setting to show more or less data. My old vista hcx had a setting that did that.